This invention relates to the use of a pigment for the warpage-free coloration of polyolefins.
Unexpected shrinkage and warpage problems can occur when organic pigments are used for the coloration of polyolefins, especially high density polyethylene. This may be due to changes in crystallization behavior due to the presence of pigments. A number of studies have shown that pigments can affect both the size of the crystallites and the rate of crystallization by acting as nucleating agents, inducing crystallization at higher temperatures than expected.
All these aspects have an important commercial issue. For example the coloration of bottle crates is a very important market for a pigment producer. A pigment has to fulfill an important condition when used in high density polyethylene application: it must not influence the crystallization process of the polymer. Such an influence can cause shrinkage or warpage, showing deformations of the injected article and deterioration of the mechanical properties.
In order to avoid this problem, when using organic pigment for the coloration of polyolefins, surface treatments have been developed in order to modify the surface of the pigment precipitation on the surface of the organic particules of a thin layer of a metal oxide (e.g. zirconium, silicium or aluminium) or adsorption or precipitation of some polar polymers (polyacrylates or cellulose derivatives) on the pigment surface. However these treatments are not always efficient.
It has now been found that light stabilizing azo-pigments of formula (I) do not provoke warpage when used in high density polyethylene 
where both R are Cl or COOCH3.
The invention, therefore, resides in the use of a pigment of formula (I) for the coloration of polyolefins which do not warp and provides warpage-free polyolefins, more preferably high density polyethylene (HDPE), which have been colored with a pigment of formula (I).
When coloring polyolefins with a pigment of formula (I) the usual amounts can be used, i.e. from 0.01 to 30% by weight, preferably from 0.1 to 10% by weight based on the material to be colored.